1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst suitable for application to treatment for purifying exhaust gas discharged from an internal combustion engine, and to a manufacturing method thereof.
2. Description of the Related Art
Up to present, as catalyst activators of a three-way catalyst capable of simultaneously purifying carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx), which are contained in exhaust gas, noble metals such as platinum (Pt), rhodium (Rh) and palladium (Pd) have been widely known. Moreover, exhaust gas purifying catalysts, in which these noble metals are supported on supports of oxides, for example, such as alumina, zirconia and titania, have been widely known. These exhaust gas purifying catalysts are formed by being coated on surfaces of inner walls of a honeycomb base member made of cordierite or the like, and purify the exhaust gas introduced into the honeycomb base member from an internal combustion engine.
Moreover, as the exhaust gas purifying catalyst, there is one to which a promoter component is added in order to enhance catalytic performance. This promoter component is, for example, an oxide of transition metal. The promoter component is added so as to contact or come close to particles of the noble metal as the catalyst activator, and thereby exerts a function as an active site, thus making it possible to enhance catalytic activity.
In recent years, on the background where outputs of gasoline engines have been enhanced and high-speed running has been increased, a temperature of the exhaust gas of automobiles has tended to rise. Moreover, at the time when such an engine starts, in order to rapidly raise a temperature of the honeycomb base member to a temperature at which the exhaust gas purifying catalyst is capable of purifying the exhaust gas, the honeycomb base member in which the exhaust gas purifying catalyst is formed is disposed immediately under the engine. From these facts, the exhaust gas purifying catalyst has come to be used in a higher temperature range than heretofore.
The conventional catalyst is poor in durability in the actual exhaust gas, and in some cases, grain growth has occurred in the noble metal itself owing to the high temperature, and the activity thereof has been decreased.
In the exhaust gas purifying catalyst added with the promoter component, the promoter component is disposed in the vicinities of noble metal particles, thus making it possible to suppress atmospheric variations around the noble metal particles by the transition metal or a transition metal compound. From this fact, an attempt to enhance durability of the noble metal particles in the actual exhaust gas has been made (refer to Japanese Patent Laid-Open Publications No. H8-131830 (published in 1996), No. 2005-000829, No. 2005-000830 and No. 2003-117393). Note that, in accordance with such measures as disclosed in the publications, enhancement of the activity of the noble metal particles can also be expected in addition to the enhancement of the durability of the noble metal particles.